Abstract
Filaments are intrusions of upwelling water into the sea, separated from the surrounding water by fronts. Current knowledge explains the enhanced primary production and phytoplankton growth found in frontal areas by external factors like nutrient input. The question is whether this enhancement is also caused by intrinsic factors, i.e. simple mixing without external forcing. In order to study the direct effect of frontal mixing on organisms, disturbing external influx has to be excluded. Therefore mixing was simulated by joining waters originating from “inside” and “outside” the filament in mesocosms (“tanks”). These experiments were conducted during two cruises in the northern Benguela upwelling system in September 2013 and January 2014. The mixed waters reached a much higher net primary production and chlorophyll a (chla) concentration than the original waters already 2-3 days after their merging. The peak in phytoplankton biomass stays longer than the chla peak. After their maxima, primary production rates decreased quickly due to depletion of the nutrients. The increase in colored dissolved organic matter (CDOM) may indicate excretion and degradation. Zooplankton is not quickly reacting on the changed conditions. We conclude that already simple mixing of two water bodies, which occurs generally at fronts between upwelled and ambient water, leads to a short-term stimulation of the phytoplankton growth. However, after the exhaustion of the nutrient stock, external nutrient supply is necessary to maintain the enhanced phytoplankton growth in the frontal area. Based on these data, some generally important ecological factors are discussed as for example nutrient ratios and limitations, silicate requirements and growth rates.
Highlights
The Benguela system is one of the four mayor upwelling regions in the world ocean (Carr and Kearns, 2003; Lachkar and Gruber, 2012), and part of the eastern boundary current of the subtropical gyre in the South Atlantic
It has to be mentioned that the timing of upwelling is different in the southern Benguela upwelling system, which is separated from the northern Benguela upwelling system by the Lüderitz upwelling cell at 26◦S (Duncombe Rae, 2005; Hutchings et al, 2009)
Three general answers of the system after such a merging are possible: (1) The organisms contained in the waters decline because they are suddenly exposed to strange conditions and may not adapt to them quickly, (2) the organisms are not influenced, (3) the organisms benefit from the new conditions if one water mass contained a resource in excess that limited the growth in the other water mass
Summary
The Benguela system is one of the four mayor upwelling regions in the world ocean (Carr and Kearns, 2003; Lachkar and Gruber, 2012), and part of the eastern boundary current of the subtropical gyre in the South Atlantic. Phytoplankton Stimulation in Upwelling Filaments layer forms the basis of intensive primary and secondary production. Both are leading to a rich biomass stock, which is of high commercial value. Upwelling events in the Benguela region off South-West Africa are driven by the prevailing south-east trade winds and the resulting Ekman offshore transport (Lutjeharms and Meeuwis, 1987). It has to be mentioned that the timing of upwelling is different in the southern Benguela upwelling system, which is separated from the northern Benguela upwelling system by the Lüderitz upwelling cell at 26◦S (Duncombe Rae, 2005; Hutchings et al, 2009)
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